The present invention claims the priority of United Kingdom patent application Serial No. 9930356.2 filed Dec. 22, 1999 and relates to a self-lubricating ballscrew comprising a screwshaft, a ballnut surrounding the screwshaft and provided with a re-circulating path for a plurality of balls which roll between the ballnut and the screwshaft when the ballscrew is in use, and lubrications means in contact with the helical groove of the screwshaft to provide lubrication therefor.
The present invention seeks to provide a remedy for one or more of these drawbacks.
Accordingly, the present invention is directed to a lubricating ballscrew as set out in the opening paragraph of the present specification, in which the self-lubricating means comprises a lubricant impregnated solid having an elongate portion which follows the path of and is received in the helical groove of the screwshaft, the cross-section of that portion being different from that of the said groove, so that the contact region between the said portion and the said groove coincides with the lines of contact between the balls and the screwshaft, and the deposit of lubricant is concentrated on those lines.
Preferably, the said portion has a section which is at least in part polygonal, for example rectangular or square, the cross-section of the groove being a curve, for example semi-circular in shape.
Advantageously, to balance the lubricating effect of the lubrication means against the friction it causes, the length of the said portion corresponds to substantially one pitch of the helical groove.
Advantageously, the lubrication means are secured to the ballnut.
At least a portion of the lubricating means may be received between the ballnut and the screwshaft.
A desired pressure of the lubricating means against the screwshaft may be obtained if the lubricating means comprise two semi-cylindrical segments, each having one end retained within the ballnut between the ballnut and the screwshaft and another end projecting outwardly from the ballnut in an axial direction along the screwshaft, the segments being urged towards one another by spring means.
Such lubricating means may be provided at both ends of the ballnut.